There have so far been proposed optical recording media, typically optical disks, including those of read-only type having information signals prerecorded therein by a pattern of pits which are microscopic holes, those having a phase-change layer to which information signals can be written, and those having a magneto-optical recording layer to which information signals can be recorded. Generally in an optical recording medium recording and/or playback apparatus that uses, as a recording medium, a recordable one among such types of optical recording media, namely, in an optical disk drive, there is provided a semiconductor laser that is relatively large in maximum optical output power rating. It should be noted that an optical disk drive designed to play only a read-only optical disk as a recording medium needs a light source not as large in maximum output rating as that of an optical disk drive which uses a recordable optical disk but which can emit a greater amount of light than a certain value.
The reason for the above will be described below:
Generally, the semiconductor laser whose output is small cannot easily provide any stable light emission and the laser noise will be larger. Therefore, to assure a CNR (carrier-to-noise ratio) of information signals when the latter are recorded in an optical disk, the optical output power of the semiconductor laser has to be set to a large level than a certain value (normally, 2 to 5 mW).
On the other hand, in case information signals are recorded to an optical recording medium to which the signals can be recorded, a light beam is focused on the recording surface of an optical recording layer of the medium to a higher temperature than predetermined. In this case, the power of a light beam from a semiconductor laser for reading the information signals from the medium should be large enough to assure an ample CNR of the read information signals, and also that for recording information signals to the medium should be large enough to assure heating of the recording layer to the temperature higher than predetermined and a stable recording of the information signals. Normally, the maximum power of writing light used for writing information signals to an optical recording medium is about 5 to 20 times larger than that of reading light. Further, for recording information signals at a higher speed than a standard one, the semiconductor laser has to provide a larger optical output power.
For the above reason, the maximum rating of optical output power of a light source used in an optical head that makes both write and read of information signals to and from an optical recording medium, or of a light source used in an optical head that writes and reads information signals to multiple types of optical recording media, is normally about 20 to 50 mW. An optical head used in an optical disk drive that rotates an optical disk at a velocity about eight times higher than a standard velocity of rotation to write information signals to the optical disk employs a light source that provides an optical output power of about 100 mW.
A light source having a large maximum rating of optical output power is difficult to implement and it will consume much power. However, in case the maximum rating of optical output power is reduced with the difficulty and great power consumption taken in consideration, a large laser noise will take place during playback of an optical disk, which will lead to a poor characteristic of reading.
On the other hand, the read-only optical disk such as DVD (digital versatile disk) has two recording layers. Also, there has been proposed an optical disk capable of recording information signals, namely, a recordable disk, and which has two or four recording layers. Write or read of information signals to or from each of the plurality of recording layers of the optical disk needs a writing- or reading optical power more than about 1.5 to 2 times larger than write or read to or from a single recording layer.
Thus, in an optical disk drive which can selectively play a single-layer optical disk and multilayer one, the ratio between the maximum optical power for writing information signals to the multilayer optical disk and read optical power for reading information signals from the single-layer optical disk will be more than two times larger than that in an optical disk drive designed to play a single-layer optical disk.
Further, if the linear velocity at which a light beam scans a recording track is different from one type to another of an optical disk being played, the optical powers necessary for write and read of information signals also are different correspondingly. That is, as the linear velocity at which a light beam scans a recording track on an optical disk is higher, the optical powers necessary for writing and reading information signals should be larger.
To assure a stable write or read of information signals to or from an optical recording medium designed to have a plurality of signal recording layers, or an optical recording medium designed to rotate at a higher velocity, both intended for an increased recording capacity and also to or from a conventional optical recording medium having a single recording layer, a light source included in an optical head should be able to provide an optical output power in a wider dynamic range.